This invention relates generally to the manufacture of electronic assemblies and more specifically to testing of the electronic assemblies to find manufacturing defects.
Traditionally, electronic assemblies, such as printed circuit boards, have been tested using an in-circuit test system. Such a system, often called a xe2x80x9cbed of nailsxe2x80x9d tester, contains a test fixture with many probes that resemble a xe2x80x9cbed of nails.xe2x80x9d The probes make contact to test points on the printed circuit board and the test system generates stimulus signals or measures responses through the probes. The goal of in-circuit testing was to make contact to enough points on the printed circuit board that the proper operation of every component could be verified and the connections between all components could also be tested.
In-circuit testing has the advantage of being very thorough and accurate. It has the disadvantage of requiring a specialized test fixture and a test program that can take a relatively long time to develop. But, it requires that there be test pads on the electronic assembly so that the probes can make contact to enough test points to run the test. As circuit boards became smaller, there was less ability to incorporate test points into the boards. Further, the cost and life cycle of products using circuit boards decreased, making it more important to develop test programs quickly. Simultaneously, testing of electronic components before they were mounted on the circuit improved, making it less important to be able to fully test the components after they were mounted to the board.
To meet the needs of low cost manufacturing of small circuit boards, a type of test equipment known as a Manufacturing Defect Analyzer (MDA) was developed. The goal of the MDA was not to test the operation of every component on the board. Rather, it was simply to verify that components were correctly attached to the board.
One type of MDA was used in conjunction with a bed of nails tester. However, the tester was configured to make contact to a reduced number of test points and the test program was simplified. DELTASCAN, provided by Teradyne, Inc of Walnut Creek, Calif., USA is one example of such a system. DELTASCAN applies stimulus to leads of a component on the board. The measured current will be different depending on whether that component is properly attached to the board. Though the probing does not indicate whether the component is properly functioning, it does indicate whether the component is present and properly attached to the board during manufacture.
The in-circuit tester could also be used in combination with non-contacting test techniques, such as FRAMESCAN sold by Teradyne, Inc of Walnut Creek, Calif., USA. This product capacitively couples a stimulus signal to a component on the board. If the component is properly installed, a predictable response can be measured at leads of that component. As with DELTASCAN, the operation of the component is not verified, but proper manufacture of the board can be verified with a simpler fixture and test program that a conventional in-circuit test.
Other approaches for creating MDAs are also used. One approach is called a xe2x80x9cflying-prober.xe2x80x9d The flying-prober uses probes like in a bed of nails tester. However, the probes are not mounted in a fixture. Rather, the probes move around, allowing one probe to contact any point on the board. As a result, only a small number of probes are needed. The probes move around, making contact to test for shorts and opens on the boards. The flying-prober does not typically have enough probes to generate and measure all the test signals needed to verify the proper operation of a component on the board. However, particularly when used in conjunction with a technique like FRAMESCAN or DELTASCAN, the flying-prober can verify that the all the components are attached to the board. It can also do limited component verification, like measuring whether a resistor has the specified resistance or a capacitor has the specified capacitance.
Another approach for creating an MDA involves the use of automated optical inspection (AOI). In an AOI system, one or more cameras take pictures of the surface of the electronic assembly. Using computerized image processing, the AOI system can verify whether the specified components are in the correct places on the electronic assembly. It can also determine whether shorts have been created by structures in places where they should not be and can sometimes detect open circuits because the solder joints do not appear as they should for a good connection.
These MDAs operate by inspecting one region of the electronic assembly at a time. To fully test the board, even if just looking for manufacturing defects, the time to test the full board will be longer than it takes to manufacture the assembly. It is undesirable for the manufacturing equipment to sit idle while the assemblies are being tested. To avoid this problem, manufacturers use various test strategies. In some cases, the manufacturer will use xe2x80x9coff linexe2x80x9d testing. As the assemblies are manufactured, they are queued up for testing. This allows the manufacture to proceed at the maximum rate possible without stopping for testing.
Another approach is to use the MDA equipment xe2x80x9con-line,xe2x80x9d meaning that the equipment tests the assemblies at the same rate that they are manufactured. However, to account for the lower rate of one MDA, multiple MDA devices would be needed to handle all the assemblies being made. More often, though, the manufacturer does not want to make a large capital investment in test equipment to fully test every assembly. Rather, the manufacturer will balance the rate of manufacture and testing by testing only portions of the assemblies manufactured. This is called xe2x80x9csampling.xe2x80x9d With sampling, it is assumed that the manufacturing process will produce similar results for boards made at similar times. Thus, all the assemblies manufactured close together in time are likely to have the same defects. Thus, if the defects are likely to appear in the sample.
One way for sampling is to do a thorough test on a board, but to skip testing of the other boards being manufactured while that test is being completed. Another way to sample is to inspect only portions of each board. As successive boards are manufactured, a different region of the board is inspected. But, every region of some board will be inspected.
A drawback of a sampling approach is that, it might take a while until a problem with the manufacturing process is detected and corrected. In this time, many tens or hundreds of assemblies might be manufactured. Those assemblies would either have to be discarded or reworked to correct the defect. Either approach is expensive and undesirable.
With the foregoing background in mind, it is an object of the invention to provide a system for manufacturing electronic assemblies that reduces the number of assemblies that must be reworked or scrapped.
The foregoing and other objects are achieved in a manufacturing operation that employs dynamic testing. A manufacturing defect analyzer is linked to an assembly device. Events in the assembly device are communicated to the manufacturing defect analyzer and cause the manufacturing defect analyzer to change the method of sampling to increase the probability that a manufacturing defect will be quickly detected.
In a preferred embodiment, the manufacturing defect analyzer is linked to a pick and place machine that installs components on the electronic assembly from reels of components. Each time a reel is changed, the specific reel changed is communicated to the manufacturing defect analyzer. The manufacturing defect analyzer will test that the components dispensed from that reel match the specified components.
In an alternative embodiment, the event causes assemblies to be routed to a different MDA while the process is verified. In one embodiment, an AOI type MDA is used for normal inspection and a flying prober is used after a manufacturing process change.